Most optical coherence tomography (OCT) systems provide either a fundus camera or a scanning laser ophthalmoscopy (SLO) channel in order to provide a 2D image of the retina, in addition to the usual OCT data. This image is usually used for alignment and for focus optimization. In an OCT-only system, i.e. without also having a SLO system or fundus camera, an SLO signal can be synthesized from the OCT signal if a 3D volume is acquired, so a 2D fundus image can still be produced.
However, in order to optimize the focus using the OCT signal, the reference path of the interferometer must be correctly matched with the object path to the patient's retina. In a spectral OCT system, light from a single source is split into two parts which traverse different paths in an interferometer. One path, the reference path, simply introduces a variable delay into the beam. The other path, called the object path, includes a patient's eye. Light scattered back from the patient's eye is mixed with light from the reference path to produce an interference signal. The interference signal is then analyzed with a spectrometer.
For an effective OCT signal to be produced the two paths must be matched in length, which is the reason for the reference path length being variable. However this can be difficult due to patient movement. If the focus is not closely matched to the patient in the first place, then the OCT signal can be very difficult to detect. The focus adjustment and the reference path adjustments have to be controlled together. The coupling of these two parameters can make it very difficult to achieve optimal focus, especially when the patient has a high diopter prescription.
There is a need to provide a method of achieving optimal focus which decouples the focus adjustment and the reference arm adjustment.